In a fabrication process for a semiconductor device such as a semiconductor memory device or a liquid crystal display, a processing apparatus (the so-called single wafer processing apparatus) processing substrates one by one is generally employed for carrying out a film forming step of forming a prescribed film on the surface of a substrate such as a semiconductor substrate or a glass substrate to be processed or an etching step. In the fabrication process for a semiconductor device or a liquid crystal display, a plurality of processing apparatuses of the aforementioned single wafer processing type are set for transporting/supplying substrates to be processed to the processing apparatuses with a moving apparatus such as a loader. A substrate holder for receiving the substrate supplied by the loader is set on each processing apparatus. Film formation or etching is performed on the substrate received on the substrate holder.
A heater is set on the substrate holder for increasing the temperature of the substrate to a prescribed level. Further, an electrostatic attraction electrode may be formed on the substrate holder, in order to attract and fix the substrate to the substrate holder. Alternatively, a method of improving flatness of the surface (substrate receiving surface) for receiving the substrate in the substrate holder for adsorbing the substrate on the substrate receiving surface may be employed for fixing the substrate to the substrate holder.
In the aforementioned substrate holder, the substrate receiving surface and portions around the same are exposed to reaction gas for carrying out film formation or etching in the film formation step or the etching step on the substrate. Therefore, the component of the substrate holder must have sufficient corrosion resistance against such reaction gas (for example, halogen gas having high corrosiveness).
In the film formation or etching of the substrate, the substrate temperature may be increased to a relatively high level. Therefore, the substrate holder is required to have sufficient heat resistance in addition to the aforementioned corrosion resistance.
Thus, in consideration of corrosion resistance, heat resistance and durability, employment of not a metal or resin but ceramic is studied as to the material for the substrate holder. Among ceramic materials, aluminum oxide, which is relatively easy to fabricate and low-priced, is put into practice as the material for the substrate holder.
However, aluminum oxide has low thermal conductivity of about 30 W/mK, and hence it is difficult to precisely control the temperature such that temperature distribution on the substrate receiving surface of the substrate holder is dispersed when aluminum oxide is employed as the material for the substrate holder. In this case, the temperature of the substrate to be processed is also dispersed, and hence it may not be possible to homogeneously perform film formation or etching on the substrate. Consequently, the characteristics of the fabricated semiconductor device or liquid crystal display are disadvantageously dispersed.
In order to avoid this problem, aluminum nitride is noticed as the material for the substrate holder. This is because aluminum nitride is excellent in heat resistance and corrosion resistance, and has a high insulation property and high thermal conductivity.
A method of fabricating a substrate holder with aluminum nitride includes the following steps, for example: First, a compact is prepared from aluminum nitride powder. A coil or a wire consisting of a high melting point metal such as molybdenum is held in this compact. The coil or the wire serves as a heater or an electrostatic attraction electrode. Thereafter the compact is subjected to hot press sintering, thereby obtaining a substrate holder. This holder is disclosed in Japanese Patent Laying-Open No. 6-76924, for example. Japanese Patent Laying-Open No. 6-76924 discloses an embedded heater structure for improving the soaking property of the substrate holder.
When a heater or an electrode is embedded in the substrate holder, power must be supplied to the heater or the electrode from outside the substrate holder. Therefore, an electrode wire connected to the heater or the electrode to outwardly extend from the substrate holder is set on the substrate holder. Further, a temperature measuring member such as a thermocouple or a sensor measuring the temperature of the substrate holder is set on the substrate holder for controlling the temperature of the substrate holder.
The electrode wire or the temperature measuring member may be corroded by reaction gas (for example, halogen-based gas) employed for substrate processing (film formation, etching, cleaning etc.). Therefore, the electrode wire or the temperature measuring member must be protected against the reaction gas in a chamber. Thus, a protective member reliably separating the reaction gas in the chamber and a region provided with the electrode wire or the like from each other is set on the substrate holder. The electrode wire or the temperature measuring member is arranged in the inner part of the protective member. This protective member must have high corrosion resistance against the reaction gas such as halogen gas and high airtightness. In particular, the junction between the protective member and the substrate holder must be subjected to airtight joining.
As a method of joining the protective member to the substrate holder in the aforementioned manner, Japanese Patent Laying-Open No. 4-78138, for example, shows a method of joining a cylindrical protective member consisting of aluminum nitride, silicon nitride, alumina or stainless steel to the back surface of a substrate holder by glass joining or brazing.
As another method of joining a substrate holder and a protective member to each other, a method of diffusion-joining a substrate holder consisting of aluminum nitride and a pipelike protective member consisting of aluminum nitride to each other by hot pressing is also known.
Japanese Patent Laying-Open No. 10-242252 discloses a technique of joining a substrate holder and a protective member consisting of aluminum nitride to each other with a joining layer mainly composed of aluminum nitride and containing a rare earth oxide, a shown in FIG. 4. FIG. 4 is a schematic sectional view showing a conventional holder consisting of a substrate holder and a protective member. Referring to FIG. 4, a ceramic base 102 including resistance heating elements 106 and a substrate base 103 and a protective cylinder 107 serving as a protective member are joined to each other with a joining layer 108 in the conventional holder 101. This gazette lists Y2O3, CeO2 or Er2O3 as a rare earth oxide contained in the joining layer 108, and states that the content of this rare earth oxide is 3 to 20 mass %.
As a technique relevant to the aforementioned technique in the point joining members consisting of aluminum nitride to each other, Japanese Patent Laying-Open No. 7-50369 discloses a technique of joining a base and a fin part consisting of aluminum nitride to each other with a joining material mainly composed of aluminum nitride. As to the composition of the joining material, the ratio of aluminum nitride to Y2O3 is exemplarily set to 97:3.
When a substrate to be processed is set on a substrate holding structure so that etching or film formation is performed on this substrate, the substrate holding structure is used under such severe environment that the temperature thereof is increased or the substrate holding structure is exposed to plasma or halogen gas employed for the aforementioned etching or the like. The substrate holding structure must maintain its strength or the like also under such severe environment. Therefore, the junction between the substrate holder and the protective member must also have sufficient corrosion resistance and heat resistance against the aforementioned plasma or halogen gas.
When foreign matter such as a trace amount of particles is present in the chamber in substrate processing, the foreign matter exerts bad influence on the substrate processing. Consequently, a failure may result from the foreign matter in the fabricated semiconductor device or liquid crystal display. Therefore, the substrate holding structure must not cause the aforementioned foreign matter. Thus, the junction between the substrate holder and the protective member is also required to have excellent corrosion resistance not to cause such a problem that the junction is damaged following substrate processing to result in foreign matter such as particles.
From this point of view, the aforementioned conventional substrate holding structure has the following problems: The metal employed for the junction in glass joining or brazing shown in Japanese Patent Laying-Open No. 4-78138 is generally inferior in corrosion resistance and has a relatively low melting point. Therefore, the metal reacts with corrosive gas such as halogen gas employed for processing the substrate, to cause foreign matter such as particles.
When diffusion-joined by hot pressing, the substrate holder and the protective member are joined to each other at a high temperature with a high load of about 9.8 to 29.4 MPa (100 to 300 kgf/cm2) applied to the junction. Thus, dimensional accuracy of the substrate holder may be deteriorated due to deformation through the hot pressing step. When such a hot pressing step is utilized, the cost for equipment necessary for fabricating the substrate holding structure or for executing the fabrication process is disadvantageously increased.
When a joining layer mainly composed of aluminum nitride and containing a rare earth oxide as in the technique disclosed in Japanese Patent Laying-Open No. 10-242252, the joining layer containing 3 to 20 mass % of the rare earth oxide such as Y2O3 as hereinabove described must be sintered at a high temperature of at least 1800° C. for joining the substrate holder and the protective member to each other. The temperature of at least 1800° C. is equivalent to a sintering temperature for forming the substrate holder consisting of aluminum nitride. Therefore, the substrate holder may be deformed due to the heat treatment under the high temperature of at least 1800° C. for joining the substrate holder and the protective member to each other with the aforementioned joining layer. In the stage of joining the protective member, the substrate holder is already provided therein with a heater or an electrode, and the shape and the dimension of the substrate holder are precisely set by machining or the like. If the substrate holder is deformed by the heat treatment for joining with the aforementioned joining layer, therefore, the shape of the finally obtained substrate holding structure is deformed. This results in a problem such that temperature distribution in the substrate holding structure differs from the designed distribution (the soaking property is deteriorated) due to the deformation of the substrate holding structure.
The joining layer mainly composed of aluminum nitride is densified by sintering. In the heat treatment for joining, therefore, no such phenomenon that the joining layer flows along irregularities of the surfaces of the substrate holder and the protective member coming into contact with the joining layer in the joined portions remarkably takes place in the heat treatment for joining. When the heat treatment for joining is performed, without particularly applying a load to the joining layer and the joined substances (the substrate holder and the protective member), therefore, such an effect that the joining layer fills up clearances resulting from the aforementioned irregularities is not much attained. The joining layer obtained in this manner is inferior in airtightness due to a large number of clearances. In order to protect the electrode wire against halogen gas or the like in the chamber with the protective member, the protective member including the joining layer must have sufficiently high airtightness (it is assumed that a helium (He) leak rate must be less than 10−8 Pa·m3/s). In order to implement such high airtightness, heating must be performed while crushing clearances of the joining layer by hot pressing in the heat treatment (the heat treatment for joining) on the joining layer. The heat treatment is performed while applying a load to the joining layer in this manner, and hence the problem that the substrate holder is deformed in this heat treatment particularly remarkably takes place as described above.
Further, mechanical or thermal stress is readily applied to the junction between the substrate holder and the protective member forming the substrate holding structure due to handling such as portage of the substrate holding structure or attachment to or detachment from an apparatus or thermal stress resulting from heating/cooling in a case of mounting the substrate holding structure on the apparatus in practice and using the same. Therefore, the junction must have sufficient strength (practically the bending strength in the junction must be at least 147 MPa (15 kgf/mm2) in four-point bending strength according to JIS).
The technique disclosed in Japanese Patent Laying-Open No. 7-50369, related to a ceramic radiator for a semiconductor comprising a base and a fin part consisting of aluminum nitride, belongs to a technical field different from that of the present invention and does not particularly require airtightness. Even if the technique disclosed in Japanese Patent Laying-Open No. 7-50369 can be applied to joining between a substrate holder and a protective member of a substrate holding structure, problems similar to those in the aforementioned Japanese Patent Laying-Open No. 10-242252 take place.
The present invention has been proposed in order to solve the aforementioned problems, and an object of the present invention is to provide a ceramic joined body and a substrate holding structure having excellent corrosion resistance and airtightness, having excellent dimensional accuracy and having sufficient durability upon application of mechanical or thermal stress and a substrate processing apparatus comprising the same.